Incline test for one design class

Discussion in 'Stability' started by motorbike, Mar 21, 2014.

  1. motorbike
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    motorbike Senior Member

    In a one design keelboat class where different builders have made hull and decks, possibly different amounts of ballast or owners have moved or altered interior furniture etc to gain an advantage, would an incline test to get the VCG and from that and the total lightship weight determine how much and where compensating weight would need to be placed to even the boats?

    Is this the right approach to level the playing field? The problem is that weight distribution has not been controlled and in addition the boats have been built in wood and glass, some balsa core and other solid layup. There is a wide variation in hull and deck weights and distribution of that weight.

    A swing test to determine weight in the ends is probably not useful as most of the sailing in done in sheltered water with little pitching. I am sure this problem has been raised in one design classes and solved, but I cant find it in the search function so my apologies if this is a repeat question.
     
  2. gonzo
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    gonzo Senior Member

    You could set an easy inclination standard. For example, a weight of 50 lb at 8' from the centerline at the mast can produce no more than x degrees of list.
     
  3. TANSL
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    TANSL Senior Member

    Upon completion of any ship, if it is not mass production of exactly the same boat, it should be, is required, an inclining test to determine lightship weight and the position of its center of gravity.
    A different thing is what Gonzo intends to check if the boat is heeled over what you would like.
    There are two things that have nothing to do.
     
  4. motorbike
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    motorbike Senior Member

    If the weight of the boats are identical then one with a lower VCG would have an advantage. What I want to determine is a way to measure and adjust the VCG if that is the appropriate action.
     
  5. TANSL
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    TANSL Senior Member

    The inclining test is the better way to determine VCG and LCG
     
  6. Ike
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    Ike Senior Member

    An inclining test will not only determine the VCG but also can be used to calculate the metacentric height (GM) , righting moment and give you a measure of stability.
     
  7. TANSL
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    TANSL Senior Member

    The metacentre is calculated without making the inclining test. Simply measure the mean draft of the ship and interpolate in hydrostatic values, boat with the current trim. Once you know the RMt (which only depends on the shapes of the boat), by inclining test the GM is calculated.
    The heeling moment is not calculated with the inclining test. Instead, we must generate a heeling enough for the ship to tilt between 2 and 3 degrees. This is precisely the first calculation to be done to raise the test: the weight to be used and the distance you have to move it to achieve the desired list.
     
  8. Eric Sponberg
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    Eric Sponberg Senior Member

    Motorbike, The fact that there has been so little control over the construction and weights of the boats necessarily renders any corrective action very problematic. Conceivably, you could to an incline test on every boat in the class, but that means that the test will have to be standardized for the class, and the rules of the test must be very strict so that the same data is retrieved from each test and is reported accurately. Then the calculations have to be done exactly the same way for each boat (best done by a single certified measurer and/or naval architect) so that the results are a true picture of each boat in the class.

    Then, once you have that information, what are you going to do with it? What is the weight spread from the lightest boat to the heaviest boat? To be as close to equal as possible, you would really have to add weight to all of the boats in the class, except the heaviest one, so that all boats are sailing at the same weight. You still have the problem of center of gravity--some of the boats will have a lower center of gravity than others, so how are you going to add the weights to all the other boats in the class, which will be different for each boat, so that all the boats have the same center of gravity? In order to assure that all the boats are as nearly identical as possible for the same performance, you will have to match weight and center of gravity of each boat. This is a logistical nightmare. Given the track record for the fact the boats weren't even built identically, you cannot expect consensus on establishing a consistent and reliable stability test AND consistent corrective action to make sure each boat is identical.

    All of this says nothing of the sail plans and rigging on the boats. How identical are they? Given the varities in hull construction, I can surmise that the rigs are going to be different, too, not much, but at least slightly. Who has the newest sails, which sail lofts have made all of the sails for the class, and what governance has there been over sail design and construction? The same applies to the running rigging and deck layouts--how identical are they? In fact, the variables in sail plan, sail design, and deck rigging layouts would likely have more impact over the performance of the boats than the variables in hull construction. And the reason I say that is because boat weight and CG location are at the same time both drag and power factors, but everything that has to do with sail design and construction along with rigging layout are practically all power factors. The devices that control power are more powerful, if you will, than the devices that control drag. Putting it another way, small changes in sail plan and sail construction--even just buying a new suit of sails--generally has a lot more effect on sailing performance than changes in boat weight and CG, all hulls being the same shape in the class.

    It's conceivable that you if you could standardize some sort of stability test for the class, you could then establish a handicapping system of some sort to distribute to the owners who then sail with that handicap in, say, minutes per mile. But how is such a handicapping system going to be devised? You may have a lighter boat with a very low center of gravity sailing against a heavier boat with a high center of gravity. What's the trade-off between the two? Your guess is as good as mine. Consider also that the heavy boat as a new suit of sails of the best technology available, whereas the light boat has 5-year-old, stretched out sails. What's the trade-off then? Again, anybody's guess.

    Since even a stability test is most likely going to result in a handicapping system, your simplest, cheapest, and best way forward is to simply rely on a PHRF type of rating for the class based on owner/skipper performance. It could be established within very narrow range limits, meaning that the class has to fall within minimum and maximum limits of PHRF rating. The rating is supposed to measure the skill of the disparate sailors in the class, which is what you are after anyway. A person with a "bad" boat is going to have to establish sailing skills that compensate for his boat, or make adjustments to his boat to correct its bad features, within the limits of the class. He should concentrate on good sails, good deck equipment, and honing up his racing skills--like all good owners and skippers do.

    That's my take on your problem--I hope it helps.

    Eric
     

  9. motorbike
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    motorbike Senior Member

    Eric thanks for your thoughtful reply,

    On some level I knew that this was not going to have a simplistic answer! The class has rules which determine hull design, sailplan, minimum weight, materials etc but is by no means comprehensive as other one designs, nor is it strictly enforced in the areas not plainly visible such as weight distribution.

    I think you are right that the way forward is to place the emphasis on a PHRF type of handicapping system.

    A lot to consider here...
     
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